Gun barrels, turbine components, internal combustion engine components, aerospace components, chemical reactors, machine tools, drilling equipment, bearings and the like are often comprised of iron, steel or other ferrous alloys. In use, such articles are frequently exposed to various combinations of high-temperature, high-pressure and corrosive ambient environments. These conditions can cause thermochemical erosion of the substrate materials leading to pitting, cratering, cracking and failure.
The prior art has recognized such problems and has attempted to prevent or minimize the erosion of ferrous materials by the use of various coatings comprised of high hardness materials. For example, U.S. Patent Application Serial No. 2002/0104588 discloses a process for extending the life of mechanical centrifuge screens by forming a layer of high-hardness iron nitride on the screen and subsequently electroplating a layer of chromium onto the nitride layer. The nitride layers of the '588 application are high-hardness layers including at least 33 atomic percent nitrogen. Likewise, U.S. Pat. Nos. 5,887,558 and 5,810,947 show coatings of high-hardness iron nitride used in connection with internal combustion engines and machine tools respectively. As will be explained in detail hereinbelow, such prior art methods have been found to be unsuitable for, and in some instances actually derogatory to, enhancing the thermochemical stability of steel and the like under high-temperature, high-pressure reactive conditions.
The present invention may be utilized to enhance the thermochemical stability of a variety of articles. For the purposes of this present discussion, the invention will be described primarily with regard to gun barrels; however, it is to be understood that the invention may be used with equal advantage in connection with any other articles which are exposed to conditions which include one or more of high-temperature, high-pressure and corrosive environments. These articles include, by way of illustration and not limitation, internal combustion engine components, turbine components, aerospace assemblies, chemical reactors, machine tools, drilling equipment, bearings and the like.
Referring now to FIGS. 1A-1C, there is shown a cross-sectional view of a portion of a gun barrel 10 of the prior art showing various stages in a process leading to its thermochemical erosion. The gun barrel 10 of FIGS. 1A-1C is typical of, and representative of, barrels associated with relatively large artillery pieces as well as small arms. The gun barrel 10 is comprised of a body of steel alloy, and a portion of this body of steel alloy is shown in these figures at reference numeral 12. It is to be understood that in some instances gun barrels are fabricated as composite members having a steel liner which defines the gun bore, and this liner is encased in the body of another material such as a body of metal or a body of a reinforced polymer.
Referring now to FIG. 1A, it will be seen that the barrel 10 includes a coating of chromium 14 deposited on the surface of its bore. This chromium layer 14 is of high-hardness and increases the wear resistance of the barrel 10. It is to be understood that in some instances, the barrel may have a layer of a different refractory material thereatop, or may not have any refractory material at all. The present invention may be used in any of these types of gun barrels. As is shown in FIG. 1A, the layer of chromium 14 includes a number of cracks 16a, 16b defined therein. These cracks pass through the layer of chromium 14 and expose portions of the surface of the underlying steel alloy 12. Also, it will be noted that a portion of the layer of chromium 14 is flaked away creating a large open area 16c which exposes the underlying body of steel 12. Cracking and flaking can occur as a result of stresses which arise when the chromium is deposited, and further cracking and flaking can occur during the use of the gun. Similar cracking and flaking can occur with other refractory layers used for this purpose.
In use, the gun barrel 10 is exposed to a high-temperature, high-pressure corrosive atmosphere created by the propellant gases generated when the gun is fired. These gases include large amounts of CO and CO2 therein together with volatile acids, sulfur-containing compounds, and the like. These reactive gases can be in the form of ions, radicals or neutral species. The cracks 16a, 16b and void 16c will permit these reactive gases to contact the underlying body of steel 12 so as to cause a chemical reaction to occur between the components of the propellant gas and the steel. For example, it has been demonstrated that CO can react with the steel of gun barrels, under firing conditions, to cause carburization of the steel. As is shown in FIG. 1B, this reaction has created carburized regions 18a-18c in the steel 12.                Carburization can adversely change the properties of the steel. For example, a typical gun steel has a melting point of approximately 1723° K; however, if the steel is carburized, its melting point drops to 1423° K. The lowering of the melting point makes carburized portions of the barrel prone to pitting and other erosion as a result of the continuing use of the barrel.        
As is shown in FIG. 1C, the carburized regions of FIG. 1B have eroded away producing pitted regions 20a, 20b, 20c in the steel 12. As will be seen, these pitted regions 20 have undercut portions of the chrome layer 14 which can lead to further cracking and flaking of that layer. In addition, the relatively rough surface of the pitted regions 20 is highly prone to further carburization and erosion. Similar reactions can also occur in engines, turbines and the like under high-temperature and/or high-pressure conditions.
Clearly there is a need for a method for stabilizing iron, steel and other ferrous alloys against thermochemical corrosion which can occur under severe use conditions. Such methods should be simple to implement and should not interfere with the function of the item. As will be explained in greater detail hereinbelow, the present invention provides a method for enhancing the resistance of ferrous materials to thermochemical erosion. The method of the present invention is unique insofar as it is a dynamic method; that is to say, it is a method which can be implemented while the article is in service. The method of the present invention does not require any pretreatment of the article, nor does it require any modification of the function or operation of the article. These and other advantages of the present invention will be described in detail hereinbelow.